Electrostatic printing



June 17, 1958 A. J. MoNcRxEFF-YEATES 2,839,400 ELECTROSTATIC PRINTING filed octqso, 195s s sheets-sheet 1 F'J 2f' j if Mmm: .ma/ea;

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d TTORNE Y June 17? 1958 A. J. MoNcmEFF-YEATES 2,839,400

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TTORNE Y June 17, 1958 f A. J. MoNcRlEFF-YEATES 2,839,400

ELECTROSTATIC RINTING Filed Oct. 50. 1953 3 Sheets-Sheet 3 W 1 Y o 7 Mmc; j? /W W jaafr #6A/744'! .mf/fai j] 2f mm u ff;

ATTORNEY- United States Patent 2,839,400 Patented .Juve .1.7, 19.58

ELECTRGSTATIC PRINTING Alexander J. lvioncriefi-Yeates, Princeton Junction, N. J.,

assignor to Radio Corporation of America, a corporation of Delaware Application October 30, 1953, Serial No. 389,347 6 Claims. (Cl. 96-1) This invention relates to electrostatic printing and more particularly to methods and means for developing electrostatic images.

An electrostatic printing process is, generally speaking, that type of process for producing a visible image, reproduction, or copy which includes as an intermediate step converting a light image or electrical signal into an electrostatic charge pattern ou an insulating surface. The process may also include the conversion of the charge pattern into a visible image which may be a substantially faithful reproduction of an original, except that it may be of a different size.

A typical electrostatic printing process may include the steps of providing a conductive backing plate having a surface coated with a photoconductive insulating material such as selenium, anthracene or sulphur, applying a blanket electrostatic charge to the surface of the photoconductive coating, focusing a light image on the charged surface in order to discharge the portions irradiated by light rays, ywhile leaving the unexposed areas of the surface in a charged condition thereby forming an electrostatic fimage. The electrostatic image is made visible by the application of a developer powder which is held electrostatically to the charged areas of the sheet. The powder image thus formed may be xed directly to the photoconductive coating, or it may be'transferred to another surface upon whic-h the reproduced image is desired and then fixed thereon. A detailed description of the steps of this process may be found in U. S. Patent No. 2,297,691, issued October 6, 1942, to C. F. Carlson.

Several different methods are presently used to render the electrostaic image visible. One method for developing the electrostatic image is to dust a developer powder on the image. By gentle blowing, the excess developer powder .is removed, leaving a visible image of powder that is 'held electrostatically to the charged areas of the image. A variation on this type of development is to blow a powder cloud across 'the electrostatic image whereby the powder in the cloud is attracted and held to the charged areas of the image. The developer powder may be coated on a smooth or knurled roller and then rolled across the electrostatic image whereby the powder on the roller is brought in lcontact with the electrostatic image, and held by electrostatic attraction.

Another `method fordeveloping the electrostatic image is to mix the developer powder with small glass beads or other suitable granular carrier material. The mixture is cascaded vacross the surface having the electrostatic image, transferrng particles vof developer powder to the charged areas of the electrostatic image. Another method of developing the electrostatic image is to mix magnetic carrier particles, such as iron, with a developer powder. This mix is carried by a magnet to form a developer brush. The brush is rubbed across the surface having the elastrostatic image, whereby the developerpowder in the brush is transferred to the charged areas of the electrostatic image, vthus forming a visible powder image.'

With the exception of the magnetic brush process, many of the processes produce a powder image that has a streaky appearance. Furthermore, these processes tend to overdevelop the edges of the charged areas while leaving solid areas relatively underdeveloped. Thus it is diicult to obtain good continuous tone prints. The magnetic brush process overcomes many of these diiculties. However, in the magnetic brush process, the ratio of developer powder to magnetic carrier particles must be maintained within a very narrow range in order to obtain the best results. lt is therefore, necessary to regenerate the brush frequently by a process which requires careful attention.

All of the above-mentioned methods of rendering visible the electrostatic image have a relatively low sensitivity. That is, the magnitude of charge in the image areas must be relatively high in order to transfer and to hold the developer powder. rthis, in turn, makes it necessary to lay down a blanket electrostatic charge of relatively high potential at the start of the process and to provide a photoconductive layer of relatively high insulating value in order to hold the charge.

One method of increasing the sensitivity is to provide a developing powder that is itself electrostatically charged. There are several methods by which developer powders obtain an electrostatic charge. Certain powders are charged because of their chemical structure. Other powders may be given an electrostatic charge by friction or triboelectric charging. The use of a charged developer powder permits the development of electrostatic image areas having relatively lower magnitudes of charge, thus increasing the sensitivity of the developing process. The diiculty with the triboelectric charging is that the developer particles are not uniformly charged and such charging is not easily controlled. Hence although the development with charged powders has advantages over development with uncharged powders, the developed image may be strealiy in appearance and tends to overdevelop the edges of the charged area of the image and to underdevelop solid areas.

An object of the present invention is to provide an improved electrostatic printing process.

A further object is to provide improved methods of and means for developing electrostatic images.

Another object is to provide improved methods of and means Ifor developing electrostatic images of high sensitivity that yields a powder image that is a substantially faithful reproduction of the original optical image.

A further object of this invention is to provide improved methods of and means for charging developer powders for electrostatic printing.

Another object is to provide improved methods of and means for producing a uniformly charged electrostatic developer powder.

The foregoing objects and other advantages may be accomplished in acc rdance with the present invention which comprises novel methods and apparatus for establishing an electrostatic image on the surface of a dielectric image sheet, bringing a uniformly charged developer substance into closed spaced relation with the electrostatic image and applying an electric field to the developer substance whereby the developer substance is attracted uniformly to and deposits uniformly upon the electrostatic image.

ent invention the developer powder is charged by applying an electric eld to a layer of powder resting on a lower electrode and is attracted charged upper electrode which may be .a at plate or a roller, or may be caught onan insulating Vsheet interposed between the lower electrode and an upper electrode.

whereby the powder becomes charged away from the lower electrode. The

By a vtirst embodiment of the pres-4 powder attracted away may be collected on an By a second embodiment the developer powder may be charged by exposure to an ion-producing or an electron source such as a corona discharge. The charged powder is brought into close spaced `relation with the electrostatic image and an electric field Vis applied to the developer powder whereby the developer powder is attracted to and deposits upon the electrostatic image. By a third embodiment, an electrostatic image is interposed between a pair of electrodes. A powder lying on the upper surface of the lower electrode is charged and attracted to the electrostatic image in a single step when a potential is applied to the electrodes. The combination of the applied electric field and the attraction of the charged deyeloper powder for the electrostatic image make it possible to vdevelop :areas of relatively low potential. The amount of powder deposited is a direct function of the magnitude of electrostatic charge on the elemental areas of the electrostatic image.

The invention will be described in greater detail by reference to the accompanying drawings in which:

Figure 1 is a sectional view of one embodiment for charging a developer powder by a method of this in- .verltifm-l Figure 2 is a sectional view of an apparatus for developing an electrostatic image using the charged powder ofl Figure 1.

:Figure 3 isasectional view of another apparatus embodiment for charging a developer powder by a method of this invention.

Figure 4 is a sectional view of an apparatus for developing an electrostatic image using the charged developer powder ofy Figure 3.

Figure 5 is a sectional view of a third apparatus embodiment for charging a developer powder by a method of this invention.

' Figure 6 is a sectional view of an apparatus for developing an electrostatic image using the charged developer powder of Figure 5. t i Figure 7 is a sectional view of a fourth apparatus for charging a developer powder by a method of this invention.

Figure 8 is a sectional view of an apparatus for developing an electrostatic image using the charged developer powder of Figure 7.

Figure 9 is a sectional view of a fifth apparatus for simultaneously charging a developer powder and developing an electrostatic image by a method of this invention.

Figure l0 is a sectional view of another apparatus for simultaneously charging a developer powder and developing an electrostatic image by a method of this invention.

Figure ll is a sectional view of a sixth apparatus for simultaneously forming an electrostatic image charging a developer powder and developing the electrostatic irnage with the charged developer powder.

Figure l2 is a sectional view of a seventh apparatus for electrostatic printing utilizing photoconductors having short relaxation times.

Similar references gures are applied to similar elements throughout the drawing.

Example 1 tial of about 60G volts with respect to ground by a voltage source 53 connected thereto through a lead 45, a voltage dividing potentiometer 61 and `a double-pole doublethrow reversing switch 673. The upper electrode 23 is broughtlparallel to andabout 1/z"awayfrom the pow- 4 der layer 31 in thelower electrode 21. The electrodes 21 and 23, acting as plates in a condenser, charge the powder particles 31 positively. The charged powder particles move upwards through the electric field, formed between electrodes 21 and 23, depositing as a thin layer 33 on upper electrode 23. When a sufficiently thick layer of charged particles has formed, which is of the order of l0 to l2 layers of particles, the negative potential and the lower electrode 21 are removed. An insulating layer 29, having an electrostatic image previously established thereon, on a second lower electrode 27 that is grounded through av lead 49, is brought parallel to and within 1A of the layer of charged particles 33 and a positivepotential, of about 100 volts with respect to ground, is applied to the Upper electrode 23. Sincepolyvinyl formal powder loses its charge rather rapidly, it is necessary to bring the electrostatic image close to the charged particles 33 within about 30 seconds after charging. On bringing the electrostatic image close to the charged powder, the combination of the electric field formed by the electrodes 23 and 27, and the attraction of the positivelycharged developer powder for the negatively-charged area of the electrostatic image, the positively-charged developer powder is attracted to and deposits upon the negatively-charged areas of the electrostatic image, thereby producing a powder image of particles 35.

Example 2 Referring to Figures 3 and 4, a lower electrode comprising a brass plate 21, held at ground potential through a lead 47, supports a thin layer 31 of finely powdered colored nylon on its upper surface. The powder layer 31 is exposed to the negative discharge from a high voltage source (not shown), for example, a corona discharge apparatus, charging the nylon powder particles negatively. An upper electrode comprising a brass roller 43, is connected through lead 4S, a voltage-dividing potentiometer 61 and double-pole, double-throw reversing switch 63, to a source 53 of potential positive with respect to ground. The roller 43 is rolled out of contact with, but close to, the charged powder layer 31. As the roller 43 passes above the powder 31, the negativelycharged powder particles 31 are attracted to the positively-charged roller 43. The powder particles 31 move across the gap and settle on the roller 43 forming a coating thereon of charged powder particles 33. Next a negative potential with respect to ground of about 200 volts is applied to the roller 43. The roller 43 is brought close to the surface of a grounded plate 27 coated with an insulating layer 29 having a positive electrostatic image previously established thereon. The roller 43 is spaced about 1/s from the insulating surface 29 which 1 is held at ground potential through a lead 49. The electric field formed as a result of the potential difference between the roller 43 and the ground plate 27 combined with the attraction of the negatively-charged particles coated on the roller 43 for the positively-charged areas of the electrostatic image, produce a suicient attraction to move the developer particles 33 from the roller 43 to the electrostatic image. The developer powderparticles 35, attracted to the positively-charged areas of the electrostatic image, from a powder image thereon.

Example 3 Figures 5 and 6v show an apparatus similar to the apparatus of Figures l and 2. Ink the apparatus of Figures l and 2, the charge on the developer powder particles 33 leaks off to the upper electrode 23 on which they are resting. Hence, the charged developer powder must be used promptly after charging. In order to store the charged powders for longer periods of time, an insulating dielectric layer 51 such as acrylic plastic, is interposed between the upper electrode 23 and the lower electrode 21. p When the powder becomes charged andh moves upwards' toward upper' electrode 23, it is intercepted by the dielectric layer 51. The powder lying on the dielectric layer will hold its charge for longer pcriods of time. An electrostatic image on an insulating layer 29 may be developed by the method described in Example l. It will be understood that the interposed dielectric layer 51 may be a separate sheet or may be a coating upon the conducting coating 23, and may be any material having dielectric properties. The higher the dielectric strength generally the longer the developer powder may be stored.

Example 4 Referring to Figures 7 and 8, a layer of developer powder 31 is placed on a dielectric coating 51 which is superimposed on a grounded lower brass electrode 23. The powder layer 31 is uniformly subjected to the negative discharge from a corona apparatus 62 connected to a high voltage source 65 through a voltagedividing potentiometer 61. The powder particles 31 thereby obtain a uniform high voltage charge. A previously established electrostatic image on the lower surface of an insulating coating 29 on the lower surface of an upper brass electrode 27 is brought parallel to and approximately 1/8 inch away from the charged powder layer 31. A positive voltage with respect to ground is applied to the upper electrode 27 by means of the voltage source 53 connected through the voltage-dividing potentiometer 61 and the double-pole, double-throw reversing switch 63. The voltage is gradually increased until the combination of the electric elds from the charged' powder 31, the electrostatic image and from the electrodes23 and 27 causes the powder particles to rise from the lower electrode to the electrostatic image, forming a powder image thereon which is a faithful visible reproduction of the electrostatic image.

Example 5 For certain purposes, it may be desirable to charge the developer powder and develop the image simultaneously. Figure 9 shows an apparatus identical to the apparatus of Figure 8 except that an uncharged powder 31, for example, black copper oxide rests on `a lower brass electrode 21. When a positive voltage isapplied to upper electrode 27, the powder 31 becomes negatively charged and is attracted to the positive areas of the electrostatic image thereby forming a powder image.

A variation of the same principle as employed in the apparatus of Figure 9 is shown in Figure l0. A developer powder 31 rests on the upper surface of a lower brass electrode 21. The lower electrode is grounded, and closely spaced thereto is an upper brass electrode 27 which is connected to the voltage source 53 through the reversing switch 63, the potentiometer 61 and the lead 45. The insulating sheet 29, having an electrostatic image previously established thereon, is separate from the upper electrode 27 and is inserted between the electrodes 21 and 27. A positive voltage, with respect to the ground electrode 21, applied to the upper electrode 27 causes the powder 31 on the lower electrode 21 to become negatively charged. The charged powder moves upwards and is attracted to the electrostatic image due to the electricY fields present, thereby forming a powder image on the copy sheet 29.

Example 6 Referring to Figure 1l, it is sometimes desirable to establish an electrostatic image, to charge the developer powder and to develop the electrostatic image in a single operation. The apparatus of Figure ll is similar to the apparatus of Figure 9 except that the upper electrode comprises a transparent electrically-conductive layer 27, for example, a layer of tin chloride coated on glass, and a layer of a photoconductive insulating material 29, for example, selenium thereon. The lower electrode 21, having an uncharged developer powder 31 on its upper surface, is grounded and a positive voltage with respect to ground is applied to the conductive layer 27. An optical image is projected i.' om above through the conductive layer 27 onto the photoconductive layer 29. The irradiated areas of the photoconductive layer permit a positive charge to form on the lower surface of the photoconductive layer. Simultaneously a negative charge, attracted to the lower electrode due the capacitance of the apparatus, charges the developer powder 31 negatively. The negatively-charged-particles 31 attracted by the electric fields between the electrodes and from the electrostatic image, move upwards and deposit upon the positively-charged areas of the photoconductive layer 29.

Ordinarily the apparatus described requires the use of photoconductors having relatively long relation times. The relaxation time a photoconductor is the period of time in which an arbitrary percent of a charge stored on it will leak olf. Photoconductors with very short relaxation times may be used in the apparatus of Figure l2. The apparatus of Figure l2 is similar to the apparatus of Figure ll except that a slidable conducting plate 71 is interposed between the photoconductive layer 29 and the lower' electrode 31. A iiexible lead 73 connects the conducting plate 71 to ground. When a voltage is applied to the conductive layer 27 of the upper electrode and an optical image projected upon the photoconductive layer 29, an electrostatic image forms on the lower surface of the photoconductive layer as previously described. This electrostatic image is held on the surface by the capacitance between the image, the lower electrode 21 and the conducting plate 71. The close spaced relation of the conducting plate 71 to the electrostatic image allows a greater charge to be stored on the photoconductor.

en the conducting plate 71 is slid out, the developer powder 31 is attracted to the electrostatic image thereby developing the image. It is only necessary for the leakage time of the charges in lighted areas be less than the response time of the developer powder which should be less than the relaxation time of the photoconductor. By this arrangement, the slightly charged areas of the photoconductive layer 29 will lose their charge before the powder responds but the powder will nevertheless develop the more highly charged areas of the image. The method using photoconductive layers with short relaxation times works equally well when other methods of image formation are used.

Almost any powder may be charged positively or negatively by the methods of this invention. However, materials that have a relatively low resistivity will lose their charge very rapidly and therefore development must be carried out very soon after charging. lt is preferred to use materials of resistivity of at least i010 ohm-centimeters. There are numerous examples of materials that have a resistivity to hold a charge suiciently long to allow developmentby the disclosed methods. Examples are: polyvinyl acetate and polyvinyl chloride resins, sawdust, magnesium silicate, bentonite, shellac, black copper xide, and glass. Low-resistivity materials that have been used successfully include graphite, iron, zinc and copper. Besides powders, finely divided liquids may also be employed.

Electrostatic images developed by the method of the instant invention have several distinct advantages. Most of the eld for attracting the developer powder to the electro-static image is provided by the separate electric tield between the electrodes. Thus, areas of an electrostatic image having relatively low potential which are not properly developed by previously known processes may now be developed more readily. Another advantage in the instant invention is that the amount of powder attracted to elemental areas of the image is a function of electric potential diiference between that area and the developer particles. The instant processes do not tend to overdevelop the edges of the electrostatic image area or 7 to leave the solid areas of the image relatively undeveloped. Hence,l subtle differences in shading or in color tones as well as lines and solid areas may be reproduced more faithfully.

lThe instant processes have the further advantage in that there is considerably less spurious deposit in the noncharged image areas of the print. One explanation is that in prior processes the developer particles were brought into contact with the image bearing surface by mechanical means. Some of the powder may be held to the surface by triboelectric or other surface forces. Since the instant process utilizes only electrostatic attraction for moving the powder particles to the image, the effects of triboelectric attraction is reduced to a minimum.

There has thus been described improved methods and means` for developing electrostatic images and for charging electrostatic developerpowders. The method for developing electrostatic images has a relatively high sensitivity and yields powder images that are substantially more accurate visual reproductions of the original optical images.

What is claimed is:

l. A method of rendering visible a pattern of electrostatic charges on an electrically-insulating sheet which comprises charging a layer of dry powder, subjecting the charged powder to a first electric field thereby attracting the powder through said first electric field, intercepting the charged powder on an intermediate surface, closely spacing said pattern of electrostatic charges from the intercepted powder, removing said rst electric field, and applying a second electric field which is reverse in direction to said first electric field between said pattern of electrostatic charges and said intermediate surface whereby the intercepted powder is attracted to and deposits in the configuration of said pattern of electrostatic charges.

2. In an electrostatic printing process, a method of rendering visible a pattern of electrostatic charges on an electrically-insulating sheet which comprises charging a layer of dry powder having a bulk resistivity of at least 1010 ohm-centimeters, subjecting the powder layer to a first electric eld thereby attracting the powder through said first electric field, intercepting the charged powder on a suitable surface, closely spacing said pattern of electrostatic charges from the intercepted'powder, removing said first electric field, and applying a second electric field which is reverse in direction to said first electric field between said pattern of electrostatic charges and said surface whereby the powder is attracted to and deposits in the configuration of said pattern of electrostatic charges.

3. In an electrostatic printing process, a method of rendering visible a pattern of electrostatic charges on an electrically-insulating sheet which comprises subjecting a layer of dry powder having a bulk resistivity of at least 101 ohm-centimeters lying on a lower electrode to a first electric field between said lower electrode and a closely spaced upper electrode thereby charging the powder and attracting the powder tothe upper electrode, closely spacing said pattern of electrostatic charges below the charged powder on said upper electrode, removing said lirst electric field, and applying a second electric eld which is reverse in direction to said rst electric field between said electrodes whereby the powder is attracted to and deposits in they configuration of said pattern of electrostatic charges.

4. kln an electrostatic printing process, a method of rendering visible an electrostatic image on an electricallyinsulating sheet which comprises exposing a layer of dry powder having a bulk resistivity of at least 101 ohmcentimeters to a corona discharge thereby charging the powder, closely spacing successive areas of a surface having the opposite polarity of charge from said powder layer thereby attracting a coating of charged powder to said surface, reversing the polarity of charge of said surface and closely spacing successive areas of said powdercoated surface from said electrostatic image whereby thepowder is attracted to and deposits in the configuration of said electrostatic image.

5. In an electrostatic printing process, a method of rendering visible a pattern of electrostatic charges on an electrically-insulating sheet which comprises subjecting a layer of dry powderhaving a bulk resistivity of at least 101"v ohm-centimeters lying on a lower electrode to a first electric field between said lower electrode and a closely spaced upper electrode thereby charging the powder and attracting the powder to the upper electrode, interposing a thin sheet of dielectric material between the electrodes, intercepting the powder moving to the upper electrode, closely spacing said pattern of electrostatic charges below the intercepted powder on said dielectric material, removing said first electric field, and applying a second electric eld which is reverse in direction to said first electric field between the electrodes whereby said intercepted powder is attracted to and deposits in the conguration of said pattern of electrostatic charges.

6. A method of electrostaitic printing which comprises applying a blanket electrostatic charge to the surface of a photo-conductive insulating material coated on a conducting sheet, projecting a light image upon said photoconductive coating thereby producing an electrostatic image, exposing a layer of dry nylon powder to negative ions from a corona dischargev thereby charging the powder, rolling a-roller having a positive polarity of about 400 volts about Ms inch away from said powder layer thereby attracting a coating of powder to the roller surface, applying a negative polarity of about 200 volts to said coated roller, rolling said coated roller about Vs inch away from said electrostatic image whereby powder coated on said roller is attracted to and deposits in the configuration of said electrostatic image, transferring the deposited powder image to a record-receiving surface, and then Xing said transferred powder image thereon.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Electrostatic Separations of Solids, F. Fraas & O. C. Ralston; Industrial and Engineering Chemistry; vol. 32, No. 5; May 1940; pages 600-604; pages 601 and 602 particularly relied upon.I 

6. A METHOD OF ELECTROSTAITIC PRINTING WHICH COMPRISES APPLYING A BLANKET ELECTROSTATIC CHARGE TO THE SURFACE OF A PHOTO-CONDUCTIVE INSULATING MATERIAL COATED ON A CONDUCTING SHEET, PROJECTING A LIGHT IMAGE UPON SAID PHOTOCONDUCTIVE COATING THEREBY PRODUCING AN ELECTROSTATIC IMAGE, EXPOSING A LAYER OF DRY NYLON POWER TO NEGATIVE IONS FROM A CORONA DISCHARGE THEREBY CHARGING THE POWDER, ROLLING A ROLLER HAVING A POSITIVE POLARITY OF ABOUT 400 VOLTS ABOUT 1/8 INCH AWAY FROM SAID POWDER LAYER THEREBY ATTRACTING A COATING OF POWDER TO THE ROLLER SURFACE, APPLYING A NEGATIVE POLARITY OF ABOUT 200 VOLTS TO SAID COATED ROLLER, ROLLING SAID COATED ROLLER ABOUT 1/8 